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Tectonophysics 459 (2008) 123–139
Contents lists available at ScienceDirect
Tectonophysics
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t e c t o
Temporal and spatial relationships of thick- and thin-skinned deformation: A case
study from the Malargüe fold-and-thrust belt, southern Central Andes
Laura Giambiagi a,⁎, Florencia Bechis a, Víctor García b, Alan H. Clark c
a
b
c
Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales — CCT-CONICET, Parque San Martín s/n, Mendoza, 5500, CC 330, Argentina
Laboratorio de Modelado Geológico (LaMoGe), Universidad de Buenos Aires, Pabellón II, Ciudad Universitaria, Capital Federal, 1428, Argentina
Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6
A R T I C L E
I N F O
Article history:
Received 5 June 2006
Received in revised form 21 December 2006
Accepted 15 November 2007
Available online 1 April 2008
Keywords:
Southern Central Andes
Malargüe fold-and-thrust belt
Thick- and thin-skinned tectonics
Inversion and thrusting
Simultaneous thrusting
A B S T R A C T
In this paper we analyse two end-member models of temporal and spatial interactions between thick- and
thin-skinned structures in a thrust front with pre-existing rift structures. In the most commonly accepted
model, a hinterland-to-foreland sequence of inversion of pre-existing normal faults is proposed. As a result,
the emplacement of shallow thrust sheets in the sedimentary cover occurs before the basement inversion in
the foreland. In the other model, basin inversion occurs early in the deformation history of the external part
of a fold-and-thrust belt, as the result of a foreland-to-hinterland sequence of inversion.
The Malargüe fold-and-thrust belt (34–36°S) formed in response to compression of the Mesozoic Neuquén
basin during Neogene to Pleistocene times. Integrating detailed structural data from the northern part of this
belt with new Ar/Ar dating, we propose a revised kinematic model of thick- and thin-skinned interaction and
define the temporal-spatial evolution of the belt. Comparison of the timing of deformation in the thick- and
thin-skinned areas strongly supports the hypothesis that the reactivation of normal faults was coeval with
the insertion of shallow detachments and low-angle thrusting along the migrating front of the thrust belt
and occurred from the foreland to the hinterland. Detachments occur at several stratigraphic horizons,
including a deep basement decóllement related to the basement-involved thrusting and shallow detachments
located within the Jurassic and Cretaceous beds. These shallow and deep detachments were coeval producing
simultaneous development of thrusts during the complex deformation of the thrust front between 15 and
8 Ma.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Many thrust belts are combinations of both thin- and thickskinned thrustings as a result of reactivation of pre-existing
anisotropies and weakness zones in the upper crust. The presence of
pre-existing rift structures widely exerts an important control on
thrust-belt geometry and evolution. However, the extent to which
these anisotropies control regional patterns and the kinematics of
deformation in a subsequently developed fold-and-thrust belt is
controversial. The manner in which thin and thick-skinned related
structures interact in time remains poorly constrained. This paper
sheds some light on these topics by analysing the kinematic evolution
of the Malargüe fold-and-thrust belt of the Southern Central Andes.
The Andes of Argentina and Chile between latitudes 33° and 36° S
are superimposed to the Triassic–Jurassic Neuquén basin. The northern part of this extensional trough comprises a series of NNWtrending depocentres (Fig. 1). At the latitude of the study area, the
Neogene geology of the Cordillera Principal is dominated by the
⁎ Corresponding author. Fax: +54 261 5244201.
E-mail addresses: [email protected] (L. Giambiagi),
[email protected] (F. Bechis), [email protected] (V. García).
0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2007.11.069
Malargüe fold-and-thrust belt (Malargüe FTB) involving the Mesozoic
rift sequences of the Atuel depocentre. The Malargüe FTB has been
classically identified as a hybrid fold-and-thrust belt with basement
thrust sheets transferring shortening to the Meso-Cenozoic sedimentary cover (Kozlowski et al., 1993; Manceda and Figueroa, 1995; Rojas
et al., 1999; Zapata et al., 1999; Silvestro and Kraemer, 2005). This
study establishes the kinematics of thin- and thick-skinned interaction and hence defines the temporal-spatial evolution of the northern
Malargüe FTB. We present the results of newly acquired field
observations, integrated with subsurface data acquired from oil
exploration. A new kinematic model, which integrates the structural
data and new Ar/Ar geochronology with previous surface data and Ar/
Ar dating, is proposed for the thrust front of the northern part of the
belt. A chronological study of the deformation has been used to test
how thin- and thick-skinned deformational zones interact. Attention
has been paid to the timing of basement fault reactivation and coeval
activation of a shallow detachment in the foreland. From these
observations we address the wider questions of the geometric
evolution and kinematics of fold-and-thrust belts and the role of
extensional structures in generating variable deformational styles.
Thus, does tectonic inversion of normal faults precede thin-skinned
deformation of the sedimentary sequence in the foreland, or does
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Fig. 1. Regional location map and morphostructural map of the Andes between 32° and 36° S. The location of the Malargüe fold and thrust belt, the northernmost sector of the
Neuquén Basin, and the Atuel depocentre in the present-day Cordillera Principal are highlighted. The box indicates the location of the study-area and Fig. 2.
basement inversion occur out-of-sequence after the emplacement of
shallow thrust sheets. Our research demonstrates that in the northern
part of the Malargüe FTB, deformation began with inversion of the rift
master fault, in the foreland, and subsequently migrated to the
hinterland with the simultaneous development of inverted high-angle
faults, thrust faults and basement short-cut and by-pass faults.
2. Tectonic setting
The tectonic setting and evolution of southern South America is
controlled by the subduction regime at the western margin of the
South American plate and the Mid-Atlantic Ridge spreading rates
along its eastern margin (Uliana and Biddle, 1988). During the
Mesozoic, the western margin was the site of an active trench, a
relatively narrow magmatic arc and a series of back-arc extensional
basins (Charrier, 1979; Uliana and Biddle, 1988; Legarreta and Uliana,
1991). The most important of these basins was the Neuquén basin,
which comprised several NNW-elongated depocentres implanted on
pre-Jurassic continental crust (Vergani et al., 1995). It was initiated as a
rift basin in the Late Triassic, when Chilean and central western
Argentina underwent extensional tectonism (Digregorio et al., 1984;
Legarreta and Uliana, 1991). Marine and continental sediments were
deposited in isolated depressions during the Late Triassic to Early
Jurassic and are presently exposed in the Cordillera Principal
(Gulisano, 1981; Uliana and Biddle, 1988; Legarreta and Gulisano,
1989). One example of these troughs is the Atuel depocentre, where
the northern part of the Malargüe FTB was developed (Fig. 1).
By the end of the Early Cretaceous, a major plate tectonic
reorganization took place (Somoza, 1998), ending the development
of the marine intra-arc and back-arc basins (Mpodozis and Ramos,
1989). Compressive tectonics along the western margin of southern
South America began in the late Early Cretaceous (Mpodozis and
Ramos, 1989; Cobbold and Rosello, 2003; Zapata and Folguera, 2005).
There is, however, no evidence of this early compression in the study
area, probably reflecting its eastern position. At the study latitude,
convergence was oblique during the Paleogene but became progressively more perpendicular to the trench during the Neogene with a
concomitant increase in convergence rate (Pardo Casas and Molnar,
1987; Somoza, 1998).
The main components of the tectonic setting of the region are a
magmatic arc along the Argentina–Chile border and a fold-andthrust belt, which goes from the Cordillera Principal (Malargüe FTB)
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to a series of uplifted basement blocks in the Cordillera Frontal. The
Malargüe FTB extends from 34° to 36°S and has developed since
Miocene times in a thick-skinned style related to tectonic inversion
of Mesozoic rift structures (Kozlowski, 1984; Manceda and Figueroa,
1995). Deformation involves pre-Jurassic basement rocks and
Mesozoic rift and back-arc basin deposits. The Cordillera Principal
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is underlain by Proterozoic to Paleozoic metamorphic and plutonic
rocks of the Cordillera Frontal uplifted by high-angle faults along its
eastern flank. The southern part of this range is uplifted by the
Carrizalito fault which dies out alongside a SW-plunging anticline
south of the Río Diamante (Fig. 2) (Kozlowski, 1984; Turienzo and
Dimieri, 2005).
Fig. 2. Simplified geological map of the Malargüe FTB, between 34°30′ and 35°00′S, showing major structural features and location of cross section in Fig. 11. The area has been divided
into two sectors: an eastern sector where the Upper Triassic to Upper Jurassic rocks crop out, and a western sector where the Lower Cretaceous to Neogene rocks crop out. Only the
major faults have been drawn. Boxes indicate location of Figs 5 and 6. Based on Kozlowski et al. (1981), Cruz et al. (1991), Scaricabarozzi (2003), Kim et al. (2005), Turienzo and Dimieri
(2005), Giambiagi et al. (2005a,b), Bechis et al. (2005), Giambiagi et al. (2008). D2, D3, D6, D8, D9, D10, D12, D13 and D14: location of Ar/Ar dating samples. B-B′: balanced cross
section of the Malargüe FTB on Fig. 11.
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3. Stratigraphic framework
The lithostratigraphic units of the Malargüe FTB are: Proterozoic to
Triassic metamorphic, plutonic and volcanic rocks which constitute
the basement of the belt; Upper Triassic to Lower Jurassic marine and
continental rift sequences deposited in the Neuquén back-arc basin;
Middle Jurassic to Cretaceous platform sequences; and Cenozoic
sedimentary and volcanic rocks.
3.1. Basement rocks
Basement rocks crop out in the Cordillera Frontal, northeast of the
study area (Fig. 2), and in the San Rafael block, east of the study area.
They consist of Proterozoic metamorphic rocks unconformably overlain by Upper Paleozoic marine black shales and continental
sandstones, intruded by Upper Paleozoic granitoids (Volkheimer,
1978). Permian–Triassic intermediate and acid volcanic rocks unconformably overlie the previously deformed rocks (Japas and Kleiman,
2004).
3.2. Neuquén basin infill
The lowermost Mesozoic sequences are Late Triassic to Early
Jurassic marine and fluvial synrift strata, unconformably deposited
over deformed basement rocks (Fig. 3). These strata crop out in the
western part of the study area (Fig. 2). The deposition of the marine
massive mudstones and shales of the Arroyo Malo Formation (Riccardi
et al., 1997; Riccardi and Iglesia Llanos, 1999; Lanés, 2005) marked the
onset of extensional activity in the rift basin. The El Freno Formation
crops out in the eastern sector of the Atuel depocentre and is
represented by braided alluvial deposits with a predominant eastern
provenance. The Puesto Araya Formation consists of slope-type fan
delta deposits (lower section) related to the braided alluvial systems
of the easterly El Freno Formation, and storm-dominated shelf
deposits (upper section) (Lanés, 2005). Off-shore shelf black claystones were conformably deposited over the marine strata of the
Puesto Araya Formation, and correspond to the Tres Esquinas
Formation of Toarcian–Bajocian age (Gulisano and Gutiérrez Pleimling, 1994). There is no evidence of faulting during the deposition of the
marine platform strata, indicating that the boundary between fluvial
and marine strata in the eastern part of the depocentre marks the end
of the extensional phase, as was suggested by Lanés (2005).
The middle Callovian to Oxfordian interval comprises clastics,
carbonates and evaporites of the Tábanos Formation and the Lotena
Group (Gulisano and Gutiérrez Pleimling, 1994). During Kimmeridgian
times, alluvial, fluvial and eolian continental clastic deposition was
controlled by normal faults (Tordillo Formation) (Ramos, 1985;
Cegarra and Ramos, 1996; Giambiagi et al., 2003a,b). These continental
deposits were followed by accumulation of calcareous shelf facies
(Mendoza Group). Aptian to Cenomanian red continental deposits
overlying these strata are associated with evaporites and marine
carbonates (Rayoso Group) and Late Cenomanian to Early Campanian
Fig. 3. Generalized stratigraphic column of the Meso-Cenozoic units exposed in the Malargüe FTB (from Gulisano and Gutiérrez Pleimling, 1994, and Legarreta and Gulisano, 1989).
Rift-related units, cropping out in the Atuel depocentre, are defined on the basis of the biostratigraphic zonation and correlation of Riccardi et al. (1997, 1999) and Lanés (2005).
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continental red beds (Neuquén Group: Gulisano and Gutiérrez
Pleimling, 1994, Riccardi et al., 1999). Subsequently, a transgression
from the Atlantic Ocean allowed the accumulation of clastics and carbonates (lower Malargüe Group: Barrio, 1990; Tunik, 2004), followed
by fine-grained Paleocene to Eocene sedimentary rocks of lacustrine
and playa lake origin (upper Malargüe Group).
3.3. Synorogenic deposits
Synorogenic sediments and volcanic and volcaniclastic rocks filling
a foreland basin are represented by the Miocene Agua de la Piedra and
Loma Fiera Formations, the Pliocene Río Diamante Formation, and
three Pleistocene coarse conglomerate units (Mesones, La Invernada
and Las Tunas Fms.). These rocks crop out in the Cuchilla de la Tristeza
range (Fig. 2) and are separated by angular unconformities. Foreland
basin sedimentation began with deposition of alluvial fan and fluvial
systems of the Agua de la Piedra Formation over an angular unconformity (Combina et al., 1994; Combina and Nullo, 2005). This unit
is composed of interbedded coarse conglomerate and sandstone
with clasts from volcanic and sedimentary rocks derived from the
Cordillera Principal (Yrigoyen, 1993). The base of this formation is
composed of andesitic clasts in a tuffaceous sandstone matrix. 40Ar/
39
Ar ages for two boulders (12.83 ± 0.10 and 13.44 ± 0.08 Ma) at the
base of the Agua de la Piedra Formation suggest that the unit is
younger than 13 Ma (Baldauf, 1997).
The Loma Fiera Formation unconformably overlies the Agua de la
Piedra Formation. This unit consists of cross-bedded tuffs containing
clasts of pumice and granite, overlain by volcanic breccia, conglomerates and tuffaceous sandstones and andesitic tuffs (Yrigoyen, 1993;
Combina and Nullo, 2000), interpreted as pyroclastic and laharic
deposits (Combina and Nullo, 2000). Conglomerates of this unit
appear to interfinger with andesite flows of the Huincan Formation
(Dessanti, 1959) and incorporate granitic and volcanic clasts from the
Cordillera Frontal, indicating that by the time the Loma Fiera
Formation was deposited the basement was already exposed. 40Ar/
39
Ar ages for two boulders (9.51 ± 0.07 and 10.68 ± 0.11 Ma) at the base
of the Loma Fiera Formation (Baldauf, 1997) imply a maximum age of
9.5 Ma. The overlying conglomerates and sandstones of the Río
Diamante Formation exhibit gradational contacts with the Loma Fiera
Formation, indicating deposition during a time of decreasing volcanic
and tectonic activity (Combina and Nullo, 1997).
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Atuel depocentre exhibits an asymmetric architecture interpreted by
Manceda and Figueroa (1995) as representing a half-graben with westfacing polarity. Elsewhere (Giambiagi et al., 2005a, 2008; Bechis et al.,
2005), we demonstrated that the principal normal faults of the Atuel
depocentre have been inverted and moreover, we documented a
detailed characterization of the depocentre architecture through the
integration of our structural analysis of rift-related faults with previous
stratigraphic and paleogeographic studies (Lanés, 2005). The depocentre comprised the Arroyo Malo and Río Blanco half-grabens (Fig. 4),
where the former is interpreted as a completely submerged sub-basin
filled with marine syn-rift strata (Arroyo Malo Fm. and lower section of
the Puesto Araya Fm.) and sag deposits (Tres Esquinas Fm.). Its master
fault, the west-dipping NNW-trending Alumbre fault, is well exposed
in the headwaters of the Alumbre creek, where it dips at a high angle
towards the west with no evidence of structural inversion at shallow
levels. In contrast, the Río Blanco half-graben was filled with continental syn-rift strata (El Freno Fm.) and sag deposits (upper section of
the Puesto Araya Fm. and Tres Esquinas Fm.), and was bounded along
its eastern margin by the NNW-trending La Manga master fault. Both
Alumbre and La Manga faults have been interpreted as pre-existing
structures reactivated during the rifting event. This reactivation would
have generated an oblique rift with WNW- and NNE-striking oblique
normal faults.
4.2. Andean deformation
During Miocene to Pleistocene times, the Atuel depocentre was
inverted and incorporated into the thrust sheets of the thickskinned Malargüe FTB (Kozlowski et al., 1993; Manceda and
Figueroa, 1995) exerting its structural architecture a profound
influence on the development of the belt. This influence is reflected
in a variety of structural styles in the study area. We identify several
trends of regional structures, significant changes in fold wavelengths
and multiple detachments (Fig. 5), indicating that the present-day
structure of the belt is controlled by major rift-related basementrooted faults. We argue that the mid-crustal weak zone above which
basement thrusting occurs was inherited from a previous midcrustal extensional flat detachment. The propagation of inverted
basement faults into the sedimentary cover generated complex
structures that are restricted to narrow belts characterized by tight
3.4. Cenozoic volcanism
The older Cenozoic igneous rocks, referred as Molles Suite Intrusives
(Groeber, 1951; Volkheimer, 1978), are composed of lower Miocene
basaltic and andesitic porphyry stocks associated with dacitic hypabyssal bodies (Baldauf, 1997), exposed in the western and eastern parts of
the Malargüe FTB. Intense volcanism in the Middle Miocene to Early
Pliocene (Stephens et al., 1991; Baldauf et al., 1992; Ramos and Nullo,
1993; Baldauf, 1997) is grouped in the Huincan Formation. This igneous
activity took place between 10.5 and 5.5 Ma (Baldauf, 1997) and
comprises basaltic andesites and andesites similar in chemistry to the
Teniente Volcanic Complex located tens of kilometres to the west (Nullo
et al., 2006). This magmatic event has been proposed by Baldauf (1997)
to have occurred during the waning stages of, or after compressive
deformation in the eastern sector of the Malargüe FTB. However, we will
show that this volcanic unit has the same age as the main episode of
deformation.
4. Structural setting
4.1. Rift architecture
The northern part of the Neuquén basin is a predominantly NNWtrending rift comprising a series of narrow depocentres (Fig. 1). The
Fig. 4. Block diagram illustrating the structural architecture of the Atuel depocentre,
where the main normal faults have been delineated. Note that the scale is approximate.
From Giambiagi et al. (2005a, 2008).
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129
Fig. 6. Geological map of the eastern sector of the Malargüe FTB. Modified after Kozlowski et al. (1981) and Cruz et al. (1991) and our own observations. A-A′: seismic line 16029 on Fig. 8.
folding and faulting. Deformation in these areas could have been
complicated by basement short-cut faults which generated several
detachment levels in the sedimentary cover. Towards the foreland
the Andean deformation developed a thin-skinned system using
incompetent layers from the Neuquén and Malargüe Groups as
detachment levels.
Fig. 5. Geological map of the western sector of the Malargüe FTB, based on new field observations and previous stratigraphical studies carried out by Lanés (2005).
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5. Spatial relationship between thick- and thin-skinned structures
The Malargüe FTB can be divided into western and eastern sectors
on the basis of palaeoenvironmental and tectonic relationships. Their
mutual boundary is defined by the NNW-trending Borbollón–La
Manga lineament, related to the La Manga master fault of the
Mesozoic rift system (Figs. 2 and 4).
5.1. Eastern sector
The eastern sector is an emergent thrust-front system, made up of
several N–S to NNW-trending thrust sheets involving Cretaceous to
Neogene strata in a thin-skinned tectonic style (Fig. 6). The oldest
sedimentary rocks involved in the deformation are Cretaceous shales,
evaporites and red beds. The stratigraphic section is dominated by
several incompetent evaporite and black shale units alternating with
competent sandstone units. At least two main décollements are
regionally developed in the eastern zone and account for the thinskinned architecture. The lowermost is located in the lower part of the
Upper Cretaceous red beds and is present in the northern part of the
study area, whereas the shallowest is recorded in the uppermost
Cretaceous beds. In the northern part of the belt, in the Río Diamante
area, a third decóllement is located at the base of the Upper Jurassic–
Lower Cretaceous black shale succession (Kim et al., 2005; Broens and
Pereira, 2005).
Three main thin-skinned thrusts have been identified in this
sector: the Sosneado, Mesón, and Alquitrán faults (Kozlowski, 1984)
uplifted from the upper decóllement in the uppermost Cretaceous
beds (Fig. 6). The Sosneado and Mesón faults uplift the Cuchilla de la
Tristeza range and are thrust-rooted into this shallow detachment.
The Mesón thrust repeats the Neogene Agua de la Piedra Formation,
and is a low-angle, west-dipping, fault with N–S trend. This fault is
associated with a hanging wall syncline, which acted as a NeogeneQuaternary foreland basin depocentre, in which thick synorogenic
deposits record the growth history of the belt. The Sosneado thrust
transposes the Paleogene units on top of the Agua de la Piedra and
Pleistocene fanglomerates (Fig. 7). It strikes N–S and dips 24° west.
The Alquitrán fault is inferred to generate an open anticline that
affects Upper Cretaceous to Neogene strata in the Cerro Alquitrán area.
Fig. 8 sketches the present-day configuration of the eastern sector
of the belt along the section A-A′ of Fig. 6, as constrained by field and
subsurface (seismic and well) data. A migrated reflection seismic
dataset constrained by well log information from the Cuchilla de la
Tristeza range was available in this study. Two interpretations of the
seismic line 16029 have been made to identify the spatial relationship
between thick- and thin-skinned structures. Interpretation A (Fig. 8A)
assumes that the inversion of the La Manga normal fault accounts for
the detachment in the cover and generation of the Mesón, Sosneado
and Alquitrán thrusts. An alternative approach is shown in interpretation B (Fig. 8B), where the shallow detachment developed in an
initial episode of thin-skinned deformation, not related to the
inversion of the master fault, and was folded in the ensuing episode
of tectonic inversion, in agreement with previous models of the
northern part of the Malargüe FTB (Pereira, 2003; Kim et al., 2005).
Both alternatives are geometrically plausible and the low resolution of
seismic lines along the border between the thick- and thin-skinned
zones does not allow us to discriminate between them. As we will see
in next sections, we favour interpretation A because of the timing of
movement of the basement and thin-skinned faults.
5.2. Western sector
In the western sector, outcropping rocks are predominantly Upper
Triassic–Lower Jurassic rift sequences overlain by Middle Jurassic to
Lower Cretaceous deposits (Fig. 5). The Upper Cretaceous and Paleogene
rocks have been eroded in this domain, and Neogene synorogenic strata
Fig. 7. The Sosneado thrust in the Cuchilla de la Tristeza range. The fault places the upper
part of the Malargüe Group on top of Pleistocene fanglomerates and it is covered by
Holocene deposits. See map on Fig. 6 for location.
were not deposited (Fig. 3). This sector has previously been studied by
Fortunatti and Dimieri (2002, 2005), who outlined several backthrusts
related to the basement involvement in the deformation. The Andean
structural pattern shows two predominant trends (Fig. 5): NNE-striking
folds and subordinate faults; and N to NNW-striking folds and faults. The
western sector is also characterized by a combination of two deformational styles with large-scale open folds and narrow belts of intense eastvergent folding and faulting (Figs. 5 and 9). Large-scale anticlines with
associated synclines suggest regional-scale basement uplift. In the
frontal part of these inferred basement-cored folds, we propose that the
displacement was mainly transferred to the sedimentary cover,
generating narrow belts of intense folding of syn-rift and post-rift strata
(Fig. 9). Broad, long-wavelength folds developed in the hanging walls of
moderate-to-high-angle reverse faults and are considered to have
formed by inversion of older normal faults (Fig. 10, A–B). Two structures,
the La Manga and El Freno faults, are interpreted as reactivated riftrelated normal faults on the basis of the highly variable thicknesses and
facies of the rift sequences (Lanés, 2005), the high cut-off angles along
the faults, the presence of antithetic and synthetic faults reactivated in a
reverse sense (Giambiagi et al., 2005b), and syn-extensional unconformities preserving the original extensional geometry.
Fig. 11 is a cross-section incorporating a projection of the
interpretation A of the seismic line 16029 (Fig. 8A). The cross-section
has been restored with a line-length balance and constant thickness
hypothesis for the sedimentary cover, and an area-balanced method
for the basement. In this section, the previously identified (Fig. 4)
three main basement faults are interpreted to be the principal
structures of the western sector. The faults propagated upwards into
the sedimentary strata, producing shortening accommodated by
thrusting at depth and by folding in the upper levels of the pile, as
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131
Fig. 8. Seismic line 16029 located in the southeastern sector of the Atuel depocentre, and its structural interpretation (see Figs. 2 and 6 for location). Time to depth conversion was
done using Ernesto Cristallini's “Pliegues 2D” program and subsurface data from the YPF.Md.NPQ.x-1 well. Middle J + K: Middle Jurassic to Cretaceous strata (Lotena Group, Tordillo
Fm., and Mendoza, Rayoso and Neuquén Groups); UpperJ + K: Upper Jurassic to Cretaceous strata (Mendoza, Rayoso and Neuquén Groups), Upper K + Paleogene: Upper Cretaceous to
Paleocene (Malargüe Group), AP: Agua de la Piedra Fm., LF: Loma Fiera Fm. and RD: Río Diamante Fm. (A) and (B): Two kinematic models for the interaction between thin- and thickskinned deformational zones. Interpretation A assumes that the inversion of the master fault accounts for the detachment in the cover. An alternative approach is shown in
Interpretation B, where a shallow detachment in the sedimentary cover developed first, before the inversion of the master fault.
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Fig. 9. A) Two different tectonic styles observed in the western sector of the Malargüe FTB: narrow belts of intense folding associated with a broad open fold. B) Interpretation of A:
Large-scale anticlines with associated synclines are interpreted as regional basement uplifts during inversion of preexisting normal faults. In the frontal part of these folds,
displacement is mainly transferred to the sedimentary cover generating intense folding in rift-related strata. See map on Fig. 5 for location.
fault-propagation folds. The areas of intense folding and faulting are
located in front of these large-scale anticlines, as in the region east of
the El Freno anticline, where the marine sag deposits are strongly
deformed by kink and box folds (Fig. 10, C–D).
The La Manga fault system is the most significant structure in the
foothills, uplifting the Lower Mesozoic sequences on top of the
Neogene synorogenic units, and has a throw of several kilometres
(Kozlowski, 1984). We interpret this fault system as comprising three
related structures, i.e., the Arroyo Blanco fault, the La Manga inverted
normal fault, and a basement by-pass fault (Fig. 11). This highlights an
important characteristic of the basement-cover interaction along the
Triassic–Jurassic master fault, where multiple basement thrusts have
been stacked along the eastern limit of the former rift basin. The La
Manga fault can be interpreted as an inverted, west-dipping, normal
fault, because rift-related Upper Triassic–Lower Jurassic rocks are
present in its hanging wall and absent in the footwall block (Fig. 8). We
infer that this fault has a convex-up geometry, cutting the basementcover interface at a high angle and progressively decreasing in dip
upwards. This geometry strongly implies the inversion of a high-angle
pre-existing normal fault by upward propagation of a steep basement
fault into the sedimentary cover. The La Manga by-pass fault has been
inferred in the seismic line (Fig. 8). It runs along the Arroyo La Manga
Fig. 10. Examples of two broad open anticlines (A and B), and narrow tightly folded belts located in front of these anticlines (C and D). See map on Fig. 5 for location.
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Fig. 11. Balanced cross section B-B′ of the Malargüe FTB at 34° 45′S. See Fig. 2 for location. The cross section shows the relationship between the western thick-skinned sector and the eastern thin-skinned sector of the belt. The palinspastic
restitution shows the location of the main normal faults developed during the Triassic–Jurassic extension. During the Neogene inversion, these structures were inverted in association with the generation of basement short-cut faults: Alumbre
short-cut fault (ASF) and El Freno short-cut fault (ESF). The inversion of the La Manga fault is inferred to be associated with the generation of the La Manga by-pass fault (LMBF).
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with a NNW-strike (Fig. 5) and overturns Mesozoic beds in the Loma
del Medio range (Kozlowski et al., 1981). The Arroyo Blanco fault crops
out in the Arroyo Blanco creek (Fig. 5), where it transposes Lower
Jurassic sag deposits over Upper Jurassic red beds and evaporites.
Open folds in the hanging wall of this moderate-to-high angle reverse
fault have been disturbed by two associated backthrusts. These faults
have previously been described by Fortunatti et al. (2004) and
Turienzo et al. (2004) as thin-skinned backthrusts and can be
distinguished in the seismic lines (Fig. 8).
The El Freno fault has been interpreted as a NNE-striking highangle, reactivated fault with an associated basement short-cut fault
(Fig. 11). An abrupt stratigraphic change (Lanés, 2005) correlates with
the boundary between areas of open folding and intense folding and
faulting (Giambiagi et al., 2008). The inversion of this fault is marked by
the broad El Freno anticline in its hanging wall (Fig. 10B). Its curved
axial plane has been interpreted to reflect the configuration of this
normal fault at depth. Associated with this thick-skinned structure,
small-scale anticlines and synclines with angular hinges (kinks and
box-folds) deform the Lower Jurassic sequences, and low-angle thrusts
formed above shallow detachments, in thin-skinned tectonic style
(Fig. 10, C–D). The steeply-dipping to overturned beds shown by the
outcrops east of the Arroyo El Freno creek reveal structural complexity.
Associated with the inversion of this fault, we have inferred the
presence of the El Freno basement short-cut fault to account for the
generation of a broad open syncline and a low-angle thrust (Fig. 11).
The Alumbre fault is an approximately 15 km-long, NNW-striking
fault with a continuous trace. It was passively uplifted in the hanging
wall of the El Freno fault, preserving the inherited pattern of extensional
structure at shallow levels. This fault is exposed in the headwaters of
the Arroyo Alumbre creek (Fig. 5). Its orientation is consistent with the
NNW-trending paleocoast and with paleocurrents ranging from SSW to
NW documented by Lanés (2005). Although in outcrop it presents no
evidence of structural inversion, its lower segment is inferred to have
been inverted during Andean compression and to be responsible for a
series of backthrusts affecting the sedimentary cover. The generation of
a short-cut fault is associated with a basement wedge and oppositely
verging cover-detached underthrusts (Figs. 5 and 11). This complex
zone may have formed as a response to buttressing against a basement
high, previously uplifted by the inversion of the El Freno fault.
We therefore infer that the tectonic evolution of the Malargüe FTB
involved both thin-skinned tectonics along several shallow detachments within the Jurassic rift sequences (western sector) and
Cretaceous strata (eastern sector) and basement involvement along
a deeper detachment which accommodated stacking of basement
thrust units. This model predicts that steep, basement-involved
thrust-ramps in the western sector migrated upsection through
cover and evolved into flats when they reached the incompetent
syn-rift strata. A combination of extensional fault inversion and
development of new basement short-cut faults accounts for the
complex structure in the sedimentary cover.
6. Chronology of deformation
In order to constrain the age of the deformation and to choose
between both interpretations of thick- and thin-skinned interaction
(interpretations A and B – Fig. 8), we analyse the timing of
deformation of the principal structures, based on structural relationships, 40Ar/39Ar dating of tectonic and post-tectonic volcanic and
subvolcanic rocks, and the age of foreland basin deposits and
discontinuities separating the different sequences (Fig. 12). Nine
volcanic rocks were sampled and studied by laser-induced 40Ar/39Ar
step-heating procedures on hornblendes and whole-rocks (Figs. 2
and 12). We integrated our data with previous Ar/Ar dating studies by
Baldauf (1997) and proposed a four-stage temporal model for thrustbelt development. The four phases are illustrated by cross-sections
that represent time-slices from 15 to 1 Ma (Fig. 13, A-E).
6.1. Inversion of the Río Blanco half-graben (15–11 Ma)
We have previously documented the La Manga thrust system as
comprising three main faults: the inverted La Manga normal fault and
an associated by-pass fault, and the Arroyo Blanco fault (Figs. 5 and 11).
A maximum age for displacement on the La Manga thrust is given by
the age of pre-tectonic subvolcanic rocks, cropping out in the Las
Bardas creek, dated at 14.48 ± 0.61 (2σ error) Ma (Fig. 5). These rocks
are folded and affected by the deformation in the hanging wall of the
fault. In the thick-skinned domain, deformation was accommodated by
movement along the La Manga fault prior to 10.84 Ma, the age of the
Cerro Tordilla post-tectonic volcanic rocks (Fig. 5). The ages of porphyry
dikes in the Río Salado area, south of the Río Atuel, assumed to be
syntectonically emplaced by Baldauf (1997), indicate that displacement on the La Manga fault took place between 13.57 ± 0.12 and 13.43 ±
0.09 Ma (Baldauf, 1997). Initial movement on the La Manga fault
therefore would have occurred between 15 and 11 Ma (Fig. 12).
We propose that contractional reactivation of the Río Blanco halfgraben began with rigid displacement of the wedge of rift deposits and
the underlying crystalline basement rocks along the La Manga fault,
being fault displacement dissipated in the cover units by folding. The
syntectonical deposition of the syn-rift strata of the Agua de la Piedra
Formation indicates that the anticline associated with the first
movement on the La Manga fault system would have formed between
15 and 11 Ma (Fig. 13B).
6.2. Breakthrough of the La Manga fault onto the sedimentary cover and
reactivation of the El Freno fault (11–9 Ma)
After the partial inversion of the Río Blanco half-graben, faults
emanating from the master fault, such as the La Manga bypass fault
(Fig. 11) broke through the entire sedimentary section and reached the
surface (Fig. 13C). The time of breakthrough is well constrained by the
age of the post-tectonic volcanics and by the angular unconformities
between the synorogenic strata (Fig. 12). The Loma Fiera Fm. strata
have filled depressions developed during the generation of the Mesón
fault showing wedge geometry and internal unconformities related to
the uplift of the La Manga fault system. The timing of thrusting of the
Mesón fault postdates deposition of the Agua de la Piedra Formation,
although was synchronous with the deposition of the Loma Fiera
Formation in its hanging wall. The angular unconformity between
these two synorogenic units (Fig. 8) indicates that this thrust developed between 10.5 and 9.5 Ma, the age of the Loma Fiera Formation
(Baldauf, 1997).
At the same time, the internal deformation of the Río Blanco halfgraben occurred through the inversion of the El Freno fault system.
The age of movement along this system, related at depth to the
inversion of the pre-existing El Freno normal fault, is determined by
the ages of pre-tectonic volcanic rocks (11.16 ± 0.28 Ma) and posttectonic volcanics of the Tres Lagunas hill (9.07 ± 0.24 Ma) (Fig. 2). This
indicates that movement along this fault was contemporaneous with
the development of the Mesón thrust and La Manga bypass thrust, i.e.,
between 10.5 and 9 Ma, and coincided with the age of emplacement of
the Cerro Blanco porphyry copper centre (10.54 Ma — Gigola, 2004)
located in its hanging wall (Fig. 5).
6.3. Inversion of the Arroyo Malo half-graben and generation of the
Sosneado thrust (9–8 Ma)
Timing of displacement along the thin-skinned thrusts has
previously been studied by Baldauf (1997). He pointed out that
several stocks were emplaced along the trace of the Sosnedo fault after
the main pulse of compressive deformation. He dated three of these
stocks (Fig. 2), Cerro La Brea (5.97 ± 0.08 Ma), Cerro Media Luna (6.52 ±
0.04 Ma) and Cerro Ventana (7.25 ± 0.32 Ma), indicating that the
Sosneado thrust had moved before 7.25 Ma (Fig. 12). Although these
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L. Giambiagi et al. / Tectonophysics 459 (2008) 123–139
135
Fig. 12. Chart showing the chronology of thick-skinned and thin-skinned thrusting in the Malargüe FTB as determined by radiometric data of pre-, syn- and post-tectonic volcanic and
subvolcanic rocks (D2, D3, D6, D8, D9, D10, D12, D13, D14), relationships of synorogenic units, angular unconformities, and crosscutting structural relationships. The terms pre-, synand post-tectonic are related to relationship between extrusion and movement along the closest fault or fold. Times of displacement along individual faults are represented by the
shaded zone. ⁎1 From Gigola (2004); ⁎2 From Baldauf (1997). Three major pulses of deformation are highlighted. See Fig. 2 for location of radiometric data.
stocks are mainly post-tectonic, there is evidence for reactivation of
the Sosneado thrust after their emplacement. In the Cerro La Brea
area, Baldauf (1997) identified brecciated zones parallel to the fault, in
the margin of the stock, and suggested that they are fault zones
generated during the reactivation of the thrust. To the south, on the
eastern slope of the Cuchilla de la Tristeza range, the thrust plane is
exposed along a petroleum platform. In this region, the Sosneado
thrust displaces the Paleogene Upper Malargüe Group over Pleistocene fanglomerates (Fig. 7). Baldauf (1997) suggested that the Laguna
Amarga stock (10.56 ± 0.04) was not affected by the Sosneado thrust.
Our alternative explanation is that the thrust was split by the rigid,
pre-existent stock into branches along its western and eastern
margins. The eastern branch is inferred to have propagated northward
to generate the brecciated zone in the Cerro La Brea area. Moreover,
seismic data indicate that the displacement along the Sosneado thrust
took place after deposition of the Agua de la Piedra Formation. Major
activity on the Sosneado fault followed deposition of the Loma Fiera
Formation but preceded that of the Río Diamante Formation, so we
conclude that it occurred between 9.5 and 7 Ma (Fig. 13D). Toward the
east, cross-cutting relationships, together with emplacement ages,
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L. Giambiagi et al. / Tectonophysics 459 (2008) 123–139
Fig. 13. Kinematic model of the evolution of the northern part of the Malargüe fold and thrust belt showing the four-phase evolution of the belt. A) Distribution of pre-existing normal
faults before compression. B) Inversion of the Río Blanco half-graben by reactivation of the basement-seated decóllement. During this time, synorogenic deposits of the Agua de la
Piedra Fm. were deposited in a newly developed foreland basin. C) Maximum episode of deformation, between 10.5 and 9 Ma, coincident with the peak of volcanism of the Huincan
Fm. (Baldauf, 1997). Several basement and thin-skinned faults are interpreted to have simultaneously moved. D) Waning of deformation with inversion of the Arroyo Malo halfgraben. The La Manga fault system was still active. E) After 8 Ma only minor deformation occurred with generation of the Arroyo Blanco fault and movement along the Sosneado
thrust.
indicate that deformation and uplift in the Cerro Alquitrán area must
have occurred after 10.42 Ma, the emplacement age of the Cerro
Alquitrán stock (Baldauf, 1997).
In the western zone, displacement on the lower part of the
Alumbre fault occurred after the uplift and generation of the El Freno
anticline, because the related short-cut thrust decapitates the
anticline. The western structures are not rotated by the El Freno
anticline and folding of earlier décollements has not been recognized.
Therefore, the Alumbre fault inversion could have been responsible for
the final uplift of the Cerro Blanco porphyry copper centre, after 9 Ma.
This indicates that the internal deformation of the Atuel depocentre
occurred after the inversion of the La Manga normal fault.
6.4. Internal deformation of the Río Blanco half-graben and reactivation
of the Sosneado thrust (8–1 Ma)
The main phase of deformation in the Malargüe FTB occurred before
8 Ma, and after that time only minor fault movements have been
identified. We infer that the Arroyo Blanco fault was generated after the
main deformation on the La Manga fault system had ended. Structural
relationships indicate that this fault has moved after the generation
of the La Manga by-pass fault, i.e., between 9 and 8 Ma. There is no
evidence of subsequent deformation in the western zone, whereas in the
eastern zone reactivation of the Sosneado and Mesón thrusts took place
after the deposition of Lower Pleistocene fanglomerates (Fig. 13E).
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7. Discussion: temporal relationship between thick- and thinskinned structures
Many fold-and-thrust belts are combinations of both thin- and
thick-skinned thrusting as a result of reactivation of preexisting
anisotropies and weakness zones in the crust. In orogenic fronts with
influence of previous rift structures, the temporal relationship
between thick- and thin-skinned deformation is currently a topic of
controversy between two kinematic models (Fig. 14 — zone C). In the
most commonly proposed model, cover detachment on low-friction
horizons in the sedimentary cover occurs before, and basement
inversion occurs afterward, as a result of hinterland-to-foreland
sequence of inversion of preexisting normal faults (Fig. 14A). In the
other model, basin inversion occurs early in the history of the foldand-thrust belt, in the thin and thick-skinned interaction zone, as a
result of foreland-to-hinterland sequence of inversion (Fig. 14B). The
main factors favouring one model or the other are the orientation and
dip of preexisting faults with respect to the superimposed compressional stress field (Sibson, 1985), the fluid overpressure (Turner and
Williams, 2004), and the strength of the frictional basal detachment
(Buiter and Pfiffer, 2003). The first model is also favoured by the
occurrence of low-friction horizons in the cover, such as the presence
of thick evaporate layers.
In the Andes of central Argentina and Chile, the first model was
postulated for the Agrio FTB (Zapata et al., 2002; Zamora Valcarce
et al., 2006), located southward of the Malargüe FTB, where hinterland-to-foreland sequence of inversion of previous normal faults is
inferred to have generated a first phase of thin-skinned deformation
followed by a thick-skinned phase in the thrust front. The second
model was postulated for the southern part of the Aconcagua FTB
(Giambiagi et al., 2003a,b) where the preexisting Jurassic normal
faults were completely inverted during the first phase of Andean
compression. In the Malargüe FTB previous studies have postulated a
classic hinterland-to-foreland sequence of inversion of extensional
137
faults, with the generation of an early phase of thin-skinned deformation in the thrust front, followed by basement inversion tectonics
(e.g., Manceda and Figueroa, 1995; Rojas et al., 1999; Giampaoli et al.,
2002; Silvestro and Kraemer, 2005; Kim et al., 2005; Broens and
Pereira, 2005).
For the inversion of the Atuel depocentre, located in the northern
part of the Malargüe FTB, we have demonstrated that inversion of
previous normal faults occurred from the master fault, in this case
located in the foreland, to the hinterland. The reactivation of the
master fault and the coeval activation of the inferred deep-seated
detachment were synchronous with the activation of shallow
detachments and low-angle thrusting in the thin-skinned area. This
indicates that the most plausible kinematic model for the northern
part of the Malargüe FTB incorporates inversion during an early
episode of compression. Our chronology of deformation in this sector
of the belt indicates that the main phase of deformation occurred
during a brief episode of important shortening, mainly between 10.5
and 8 Ma, when displacement occurred simultaneously on several
major faults detached from different decóllement levels.
8. Conclusions
The Malargüe FTB study yields insight into fold-and-thrust belt
evolution. It illustrates the progressive evolution of the thrust front
and the synchronous movement on a number of thrust sheets. The
question whether shortening in the basement occurred first and was
transmitted to the cover, or the cover detached first and basement
thrusting occurred afterwards, has been elucidated through pre-, syn-,
and post-tectonic relations among volcanics and subvolcanic rocks,
structural relationships and foreland basin deposits. Comparison of
the timing of deformation in the thick- and thin-skinned deformational areas strongly supports the hypothesis that the reactivation of
normal faults was coeval with the activation of shallow detachments
and low-angle thrusting at the thrust front of the Malargüe FTB. Low-
Fig. 14. Two kinematic models for the temporal relationship in the interaction zone (dashed box C) between thick- and thin-skinned deformations in fold and thrust belts influenced
by the presence of preexisting normal faults. A) Cover detachment on low-friction horizons occurs before, and basement inversion occurs afterward, as a result of hinterland to
foreland inversion of preexisting normal faults. B) Basin inversion occurs early in the history of the fold and thrust belt, in the thin and thick-skinned interaction zone, as a result of
foreland-to-hinterland sequence of inversion.
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L. Giambiagi et al. / Tectonophysics 459 (2008) 123–139
angle thrusts interacted with high-angle faults related to inversion of
basement normal faults inherited from the extensional history of the
foreland, indicating a mechanics of deformation characterized by
superimposed shallow and deep detachment tectonics. Along the
thrust belt, detachments occur at several stratigraphic horizons: a
deep basement detachment related to the basement-involved thrusting, and shallow detachments located within the Jurassic and
Cretaceous sequences. We propose that these detachments were
active during the complex deformation of the thrust belt, between 15
and 8 Ma with a peak of deformation between 10.5 and 8 Ma.
Acknowledgements
This research was supported by grants from the Agencia Nacional
de Promoción Científica y Tecnológica (PICT 07-10942) and CONICET
(PIP 5843). We wish to thank Julieta Suriano, José Mescua, Maisa Tunik,
Carla Terrizzano and Marilin Peñalva for their help in the field. Special
thanks are due to Silvia Lanés for discussions and comments. The Ar/Ar
analyses were carried out by L. Giambiagi in the Geochronology
Laboratory at Queen's University, with the assistance of J.K.W. Lee and
D.J. Archibald, and funded by N.S.E.R.C. grants to A.H. Clark. Thierry
Nalpas and Tomás Zapata are sincerely thanked for their critical and
helpful reviews.
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